1. Field of the Invention
The present invention relates to a ball end mill in which cutting edges at the forward end thereof describe a semi-spherical path on a rotation.
2. Related Art
FIGS. 5 and 6 depict a conventional ball end mill, the body 1 of which is circular in cross section and rotates about the center axis C thereof. At a forward end of the body 1 which is generally semi-spherical, four cutting inserts 2 are installed.
The cutting inserts 2 are made of, for example, a sintered CBN compact (sintered cubic boron nitride carbide compact) and formed as plates shaped like a boomerang. On the outer side of each of the cutting inserts 2, a cutting edge 2a is formed. The cutting inserts 2 are evenly disposed on the body 1, and angularly spaced at 90.degree. about the center axis of the body 1. Each opposite pair of cutting edges 2a are disposed in a similar radial arrangement so that the whole of the cutting edges 2a define a semi-spherical path on rotation of the body 1.
Such a ball end mill is advantageous for high speed cutting because of the resistance to wear of the sintered CBN carbide cutting insert 2.
However, mainly during transverse traveling of the ball end mill, the forward end of the cutting insert 2 which is close to the center axis C receives a large thrust load because the rotating velocity thereat is low compared with more peripheral portions, so that the cutting resistance at the forward end is relatively high. Consequently, the cutting edges 2a at the forward end which are made of sintered CBN carbide are frequently damaged as the CBN carbide material is relatively brittle.
Especially, if a plurality of cutting edges are disposed so as to converge at the center axis C, each of the cutting edges 2a come into contact with the others at the center axis C in such a manner that the edges do not continue an unbroken line. Consequently, thickness T of the cutting edges 2a at the center axis C must be made thinner in order to achieve smooth cutting. Therefore, an increase in the number of cutting edges 2a is accompanied by greater tendency for damage to occur at the cutting edges.